Airgap armature coils and electric machines using same

ABSTRACT

A flywheel energy conversion device provides highly efficient conversion between kinetic and electrical energy. The flywheel produces increased output by providing armature coils in an air gap formed about the flywheel (both radial and axial embodiments are described). In preferred embodiments, field coils of a magnetic circuit are energized with DC drive current that creates homopolar flux within a rotating solid rotor having teeth cut from a flat disk. The total reluctance of the magnetic circuit and total flux remain substantially constant as the rotor rotates. The flux may travel radially outward and exit the flat disk through the teeth passing across an armature air gap. Airgap armature coils are preferably utilized in which the changing flux density (due to the rotating teeth) induces an output voltage in the coils. The flux is diffused before returning to the rotor in one of several ways such that core losses are effectively reduced, thereby enabling the flywheel to operate efficiently at high frequencies.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.08/597,008, filed Feb. 5, 1996, now U.S. Pat. No. 5,731,645, which isincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to flywheel energy conversion devices thatinclude motor-generators and methods for providing increased outputpower, and more particularly toward flywheel energy conversion devicesincluding brushless motor-generators having low inductance armaturewindings. The armature windings of the present invention are located inan air gap of an unusually high reluctance field circuit including largeair gaps in place of traditional armature windings that are enclosed inthe high permeability parts of a lower reluctance field circuit.

One area where flywheel energy storage devices may prove advantageous isin situations requiring a continuous supply of reserve power in theevent of a primary power source failure (i.e., failure by a utilitycompany supply). In such situations, it is often required that asecondary power source supply a nominal amount of power for a certaintime period so that various pieces of equipment utilizing primary powermay be shut down in a relatively normal fashion, rather than theinstantaneous shut down that would occur from a loss of primary powerwithout a backup supply. A traditional approach to resolving thisproblem is the use of a bank of chemical batteries, often combined withan emergency generator.

For example, in a paper mill, substantially liquid paper pulp is sprayedonto a rotating wire mesh and then carried through a long series ofrollers through ovens to remove the moisture from the pulp. It may takeseveral minutes for the liquid pulp to pass through all of the ovensbefore the pulp has dried and reached the end of the line where it isrolled up onto high speed spools. An instantaneous loss of power undersuch circumstances would be catastrophic. Therefore, paper mills oftenhave one or more large rooms filled with chemical batteries to providebackup power to keep all of the equipment running while the pulp supplyis shut off and the remainder of the pulp already on the production lineis processed.

Chemical batteries, however, suffer from various deficiencies includingbulkiness, lack of reliability, limited lifespan, high maintenance costsand relatively low safety. For example, chemical batteries requirerelatively constant and complex recharging, depending on the type ofbatteries involved to insure that the batteries continue to operateefficiently and maintain their full storage capacity. Additionally,chemical batteries raise various safety considerations due to thegeneral nature of the large quantities of caustic chemicals involved.Typical large battery installations often require special venting andair-conditioning systems for the dedicated battery storage rooms.

In order to provide an efficient replacement for chemical batteries,flywheel energy storage devices must operate at high levels of energyconversion efficiency. Thus, flywheel devices are often designed tooperate in a vacuum so as to minimize the energy losses due to air dragfriction (e.g., see Benedetti et al. U.S. Pat. No. 4,444,444). Thevacuum condition demands that heat generation in the rotating componentsbe minimized because rotor heat in a vacuum can only be dissipated byradiation or conduction through bearing surfaces which are small andhave limited heat conducting capacity. In addition, brushes used totransfer current between stationary components and rotating componentsin vacuum conditions are subject to more destructive arcing than brushesoperating in air. This essentially limits the energy storage device tobrushless operation because brushes tend to exhibit extremely shortlifespans when operated in vacuum conditions. The use of brushlessmotor-generators in flywheel storage devices is complicated, however, bythe fact that brushless motor-generators typically utilize heatgenerating components such as rotating rectifier assemblies and rotatingcoils, as described below.

The use of brushless generators is well known throughout variousindustries. For example, automobile manufacturers often utilizebrushless generators to provide electrical power to vehicles. Priorbrushless generators suffer from a variety of problems that make thempoor candidates for use with flywheel energy storage devices. Many ofthese prior generators utilize bent-over teeth as magnetic fingers inthe rotor assembly. For example, Godkin et al. U.S. Pat. No. 4,611,139and Farr U.S. Pat. No. 4,654,551 both disclose brushless alternatorsthat include magnetic bent-over fingers to produce varying magnetic fluxin the stator core. The bent-over teeth in these devices are simplyinappropriate for flywheel applications because the high speeds at whichthe tip or the flywheel must rotate would cause high stressconcentrations at the bend in the teeth which severely compromiseoperational safety. To maintain safe operations in view of the highstress concentrations, known flywheel devices often operate at lowerrotational speeds that, unfortunately, result in less stored energy fora given volume.

Another kind of brushless generator operates by applying a small inputsignal to an exciter winding that induces a much larger signal in arotating member. The input signal, which may be a DC current or a lowfrequency AC current, causes an AC current to be induced in the rotatingmember. The AC current is then converted to DC by a rectifier assemblytypically located within the rotating member, as is known in the art(e.g., see Pinchott U.S. Pat. No. 5,065,484). The rectified DC currentflows through the main windings (on the rotating member) and creates alarge rotating magnetic field. The rotating field interacts with themain armature to generate a large AC signal in the armature windings.This large AC signal, which is delivered to the external load, may beeffectively 10,000 times greater than the signal that was input to theexciter.

In some instances, the exciter may itself be excited by a permanentmagnet generator (PMG). One known example of an alternator whichutilizes PMGs is described in Farr U.S. Pat. No. 4,654,551. Farr'smagnetic flux field is generated by a rotating permanent magnet ring anda toroidal control coil, where the toroidal control coil is mounted toadd or subtract in the magnetic relationship with the ring. Farr,however, may experience potentially severe core losses due to the natureof the stationary iron core armature device.

As with most known electro-magnetic devices, many brushless generatorsare typically manufactured using iron cores in both the exciter and mainarmatures. For example, Giuffrida U.S. Pat. No. 4,647,806 describes abrushless alternator having an exciter armature formed from a laminatedstack of steel plates, and Mallick et al. U.S. Pat. No. 4,385,251describes an inductor-alternator having armature coils wound aroundslots cut into laminated stack stators. While both Giuffrida and Mallickdescribed improvements over known devices at the time, both patentsrepresent machines that, unfortunately, produce various energy losses(e.g., core losses) and have high armature inductances resulting inlimited power density.

Another necessary consideration in designing flywheel devices relates tothe negative effects of weight of the rotor. The weight of the rotor isparticularly relevant in energy storage applications--flywheel rotorstypically weigh hundreds of pounds--because the rotor must rotate atexceedingly high speeds in order to store kinetic energy. As such, themechanical bearings supporting the rotor are often placed under highstress resulting in rapid bearing wear.

One known method for addressing bearing wear in flywheel applications isthe replacement of the conventional bearings with magnetic bearings. Forexample, Benedetti et al. U.S. Pat. No. 4,444,444 describes amagnetically suspended flywheel that employs a double electromagnet anda servoloop for restoring equilibrium to a levitated rotating member.The electromagnets, which are attached to a stationary shaft, interactwith permanent magnets and a mobile armature attached to the rotatingmember to provide a magnetic attraction "equal to the force of gravity"acting on the mass of the rotor. Such a solution is relatively complex,requiring the attachment of several additional components to thestationary and rotating parts of the device.

Additionally, applications such as Benedetti often require "air-core"armature coils because an iron core armature would cause magneticinstability by competing with the stabilizing magnetic forces of themagnetic bearing. However, such devices also require a very large volumeof expensive permanent magnet material for the rotating member that isoften structurally complex to implement (e.g., Benedetti's armaturecalls for twelve rotating magnets having successively opposite polesfixed about at least one of two rings). Further, implementations such asBenedetti essentially have limited output power due to physicalconsiderations (Benedetti discusses a practical embodiment in which a370 kg rotor provides up to 10 kw of power).

Another consideration that must be accounted for when implementingelectrical machines is the negative effects of eddy currents in theunlaminated materials frequently used as part of the flux carryingmagnetic circuit. For example, Mallick et al. U.S. Pat. No. 4,385,251provides a flux shield in the form of, for example, a conducting ringconcentric with the rotor, to help prevent time varying fluxes frominducing eddy currents in the rotor steel and unlaminated back ironbecause such eddy currents lead to performance losses in the machine.However, Mallick also notes that eddy currents are induced in the fluxshields resulting in losses, but indicates that the losses are reducedwhen compared to machines without the flux shield.

In view of the foregoing, it is an object of this invention to providean improved flywheel energy conversion device that efficiently provideshigh output power, including a compact design resulting in a high powerdensity.

It is also an object of the present invention to provide an improvedflywheel energy conversion device that includes a brushless generatorfor use in vacuum conditions, where a minimum of power is dissipated inthe rotating frame.

It is a further object of the present invention to provide an improvedflywheel energy conversion device that may be safely operated atsubstantially high rpm.

It is an additional object of the present invention to provide methodsand apparatus for reducing the effects of core losses on high speedflywheel energy storage devices.

It is a still further object of the present invention to provideimproved flywheel energy conversion devices that may be produced at lowcosts when compared to currently known technologies.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished in accordancewith the principles of the invention by providing various flywheelenergy conversion devices. The preferred embodiments include a brushlessgenerator having its armature coils located in an air gap in place ofthe traditional ferrous armature core. The energy storage devices alsoinclude at least one stationary annular field coil that, in conjunctionwith a rotating toothed-rotor and a stationary laminated ring, producesflux having varying flux density (due to the gaps between the teeth onthe rotor). Airgap coils provide reduced inductance (because the coilsare radially thin) that permits faster current rise times and thus,higher power at the high frequencies that are typical of flywheeldevices. Inductance is also reduced from the flux compression thatoccurs because the solid rotor teeth are very close to the airgap coils,and because the field circuit has relatively large air gaps.

A further advantage of the present invention is related to the smoothlaminated ring that provides a site for flux diffusion that greatlyreduces changing flux in the rotor and unlaminated stationary parts,thereby reducing core losses. The armature coils are located on thesurface of the smooth laminated ring so that the coils fully link thechanging flux before any flux diffusion takes place. The use of airgaparmature coils enables the energy storage device to be produced in asubstantially compact manner while efficiently producing high outputpower.

In various preferred embodiments, an annular field coil is disposedabove and below the rotor (i.e., two annular fields coils are used) thatproduce airgap flux traveling radially outward from the axis of therotor. In these embodiments, the solid disk design of the rotor enablesthe flywheel devices of the present invention to store more inertialenergy than any design consisting of a similarly sized rotor utilizing adisk mounted on a shaft inserted through a hole bored into the disk, orany design using an annular rim joined to a shaft by radially orientedspokes. This is due, at least in part, to the fact that the maximum hoopstress in a solid disk, which limits the flywheel's maximum angularvelocity and hence its maximum kinetic energy, is at most only one-halfthe maximum hoop stress of an annular design for a given diameter, rpmand material, provided the material is homogenous. This principle isalso applicable to other embodiments of the present invention where asolid, unitary piece of metal is formed into a disk having an extendedshaft-like lower portion.

In the dual field coil embodiments, the flux, upon reaching the rotorprotrusions, passes through the stationary armature coils, then throughone or two laminated rings (i.e., a single ring or, one upper ring andone lower ring) before entering the solid steel outer shell of thedevice. The outer shell directs the flux radially inward where it passesthrough an axial gap back into the rotor. The field coil is driven by asubstantially DC current that produces a total flux density proportionalto the current. The use of DC enables the flux to be produced withsubstantially no hysteresis or eddy current losses in the rotor.Depending on the desired performance parameters, the outer shell may beformed of upper and lower rings separated by a non-magnetic ring (suchthat no flux travels vertically through the shell), or it may be formedof upper and lower shells, two axially polarized annular permanentmagnets and an annular ring (as is described in more detail below).

Another preferred embodiment includes a single stationary annular fieldcoil located on one side of the toothed portion of a rotor (while thesingle field coil is shown in the figures to be below the rotor, personsskilled in the art will appreciate that the field coil may instead, belocated above the rotor). Extending radially from the rotor protrusionsare airgap armature coils, a stationary laminated ring and a unitarysteel shell. Similar to the above-described operation, a substantiallyDC current is applied to the field coil that produces flux. In thisinstance, the flux travels axially through the rotor shaft until itreaches the toothed portion of the rotor, at which point the flux beginstraveling radially. The flux traverses the armature air gap, passingthrough the airgap armature coils, and then passes through the laminatedring and enters the steel shell. The steel shell directs the flux firstaxially, then radially back toward the shaft portion of the rotor. Thisembodiment may also be varied with the addition of permanent magnets,either radially polarized or axially polarized, depending on theinstallation location.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway perspective view of a conventional homopolarinductor-alternator;

FIG. 2 is a longitudinal cross-sectional view of a preferred embodimentof a flywheel energy conversion device constructed in accordance withthe principles of the present invention;

FIG. 3 is a cross-sectional top view of the flywheel energy conversiondevice of FIG. 2, taken from line 3--3 of FIG. 2;

FIG. 4 is a three-dimensional perspective view of a preferred airgaparmature coil constructed in accordance with the principles of thepresent invention;

FIG. 5 is three-dimensional cutaway view of the shell of an alternateembodiment of a flywheel device in accordance with the principles of thepresent invention that shows one preferred configuration of the airgaparmature coils;

FIG. 6 is a longitudinal cross-sectional view of an alternate embodimentof a flywheel energy conversion device constructed in accordance withthe principles of the present invention;

FIG. 7 is a longitudinal cross-sectional view of another alternateembodiment of a flywheel energy conversion device constructed inaccordance with the principles of the present invention;

FIG. 8 is a longitudinal cross-sectional view of a single field coilalternate embodiment of a flywheel energy conversion device constructedin accordance with the principles of the present invention;

FIG. 9 is a longitudinal cross-sectional view of an alternate embodimentof a single field coil flywheel energy conversion device constructed inaccordance with the principles of the present invention;

FIG. 10 is a longitudinal cross-sectional view of another alternateembodiment of a single field coil flywheel energy conversion deviceconstructed in accordance with the principles of the present invention;

FIG. 11 is a longitudinal cross-sectional view of another alternateembodiment of a flywheel energy conversion device constructed inaccordance with the principles of the present invention;

FIG. 12 is a longitudinal cross-sectional view of an alternateembodiment of a flywheel energy conversion device constructed inaccordance with the principles of the present invention in which fluxpasses axially through the airgap armature coils;

FIG. 13 is a partial cross-sectional top view of the flywheel energyconversion device of FIG. 12, taken from line 13--13 of FIG. 12;

FIG. 14 is a longitudinal cross-sectional view of a representativeinstallation of any of the flywheel energy conversion devices of FIGS.8-11 constructed in accordance with the principles of the presentinvention; and

FIG. 15 is a schematic block diagram of an uninterrputable power supplysystem in accordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a conventional homopolar inductor-alternator machine 1.Machine 1 includes a is stationary field coil 2 positioned between twostationary lamination stacks 3 and 4. Lamination stacks 3 and 4 haveinner surface axial slots that armature windings 9 are mounted within.An outer shell 5 (or back iron) that is typically a substantially solidpiece of steel surrounds the stator assembly and provides a flux returnpath as is described below. Mounted within shell 5 is a rotor 6 thatrotates freely. Rotor 6 has poles extending radially at each end of therotor, such that the poles 7 rotate within lamination stacks 3 and 4. Asshown in FIG. 1, the poles may be oriented such that they are offset by180 degrees.

Machine 1 is operated by applying a direct current to field coil 2. Thecurrent drives a homopolar magnetic flux through one of laminationstacks 3 and 4 and into the corresponding poles of rotor 6. The magneticflux is said to be homopolar because there are no flux reversals inindividual laminated stacks 3 and 4 as rotor 6 rotates within shell 5.Upon entering the rotor 6, the flux travels axially through the rotor 6until the other set of poles is reached. The flux then travels acrossthe air gap between the rotating poles and into the other one oflamination stacks 3 and 4. After passing through the other laminationstack, the flux completes the magnetic circuit by traveling throughshell 5 until it completes a full closed loop. It should be noted thatthere are large magnetic slots 8 between the poles 7 of rotor 6. Theseslots interrupt the flow of the flux at the air gap causing the fluxtherein to vary with time. The time varying flux generates an AC voltagein the armature windings 9.

Referring to FIG. 2, a preferred embodiment of a flywheel energyconversion device 10 in accordance with the principles of the presentinvention is described. Flywheel device 10 includes a substantiallyplanar disk rotor 12 having a series of substantially similar teeth 14(i.e., protrusions) cut out around the circumference of the disk (asindicated by hidden line 16--FIG. 3 shows a top view of the rotor 12)located within a shell 18. Shell 18 is preferably formed fromsubstantially high permeability material such as steel. Shell 18includes an upper shell 20, a lower shell 22 and an annular insert ofnon-magnetic material 24 located therebetween. While the "split" shellshown may be used for shell 18, persons skilled in the art willappreciate that shell 18 may also be formed from two pieces of highpermeability material without the non-magnetic insert to form a "single"shell. The differences between the two methods will be apparent from thediscussion that follows. In either configuration (i.e., split shell orsingle shell), a pair of axial air gaps 26 and 28 are formed betweenshell 18 and rotor 12.

Rotor 12 is preferably formed with a shaft 34 (for mounting the rotor onbearings (not shown)) from a single piece of high permeability materialsuch as steel. The use of a unitary rotor (with no center hole), whilerequiring additional weight, provides flywheel device 10 withsignificantly improved safety and performance characteristics (due tothe relative simplicity of construction and the increased safe speed ofrotation resulting in higher energy density). Mounted within shell 18are upper annular field coil 30 and lower annular field coil 32, both ofwhich remain stationary with respect to the rotor. Field coils 30 and 32are preferably configured such that teeth 14 of rotor 12 pass completelybetween the coils (e.g., the radial length of each tooth 14 is less thanthe radial length of field coils 30 and 32, and the minimum radius offield coils 30 and 32 is less than the minimum radius of each tooth 14).Although this configuration is preferred because it provides a compactdevice, the field coils 30 and 32 may be located anywhere within theshell 18 as is convenient for the required design (all that is requiredis the proper number of amp-turns necessary to induce the required fluxin the rotor for a given output power).

Flywheel device 10 may also include upper and lower bearing coils 36 and38, respectively, in which case bearing coils 36 and 38 are mounted tofield coils 30 and 32 such that bearing coils 36 and 38 are physicallybetween field coils 30 and 32 and rotor 12. Bearing coils 36 and 38, ifinstalled, may be controlled to function in the same manner as aconventional axial magnetic bearing or to relieve a set of mechanicalbearings (not shown) of a majority of the weight of rotor 12. As withthe field coils 30 and 32, the specific location of the bearing coils isnot critical. For example, the field coils and the bearing coils couldbe swapped in the embodiment shown in FIG. 2 without resulting in anynegative operational effects.

One or more strain gauges (not shown) may be attached to shell 18 tocontrol bearing coils 36 and 38. The strain gauges would provide inputsto a closed-loop controller (not shown) that commands the coils 36 and38 to increase the flux density in the upper air gap 26 by a givenamount while lowering the flux density in the lower air gap 28 by thesame amount. The net affect of this variation is to support a majorityof the weight of the rotor while maintaining an optimum constant load onthe mechanical bearing (not shown). The fact that the rotor is a solidmagnetic component enables the bearing coils 36 and 38 to operatedirectly on the rotor (rather than on a separate steel rotor as inpreviously known composite flywheel systems).

Persons skilled in the art will appreciate that, although the inclusionof bearing coils 36 and 38 is one convenient way to control the axialforces applied to rotor 12 across gaps 26 and 28, it is also practicalto omit bearing coils 36 and 38. In such circumstances, the axial forcesacross gaps 26 and 28 could be controlled by driving field coils 30 and32 separately, in which case, the net force on rotor 12 is a function ofthe difference between the currents in the separately controlled fieldcoils 30 and 32.

A pair of upper and lower smooth laminated rings 40 and 42,respectively, are also located within shell 18 (the rings are referredto as "smooth" because the surface facing the rotor is smooth whencompared to prior art laminated rings in which slots or holes are cutfor armature coil placement). Persons skilled in the art will appreciatethat a single laminated ring may be used instead of the pair of ringswhen the single shell configuration is selected. If the single laminatedring is used and no separator ring 24 is installed, shell 18 is simplyinterrupted where the separator would have been. The result is that someof the flux driven by bearing coil 36 goes through gap 28 and slightlymore power is required for the bearing function.

Laminated rings 40 and 42 may be made from laminated stacks of rings orarcs segments of high permeability material such as soft iron or steel(such that the material is only magnetized in the presence of anexternally applied magnetic field). Alternatively, rings 40 and 42 maybe formed from a solid high permeability material such as ferrite, orany other suitable material. Rings 40 and 42 are mounted within shell 18such that they are located between the radially outermost edge of rotor12 and shell 18. Depending on the desired operating parameters, rings 40and 42 may be located directly adjacent shell 18 or, instead, may belocated radially inward of shell 18 such that an additional air gap 54is formed between shell 18 and rings 40 and 42. Additional air gap 54improves the flux diffusion in the laminated ring, thus allowing it tobe smaller (which provides the additional advantage of making it lessexpensive). The additional air gap 54 also further increases thereluctance of the field circuit, decreasing the inductances and timeconstant of both the field and armature coils.

Several airgap armature coils 44 are located within an armature air gap46 that is formed between rotor 12 and laminated rings 40 and 42. Inpreferred embodiments of the present invention, airgap armature coils 44are Z-shaped coils that take up very little axial space (in the radialembodiments), as shown in greater detail in FIG. 4. As shown in FIG. 4,each armature coil 44 includes a pair of vertical legs 41 and 43 thatare exposed to flux passing through rotor protrusions 14. Airgap coils44 are preferably formed into Z-shapes such that vertical legs 41 and 43are integral with cross members 45 and 47 to form complete coil loops.Leads 49 are formed from armature coil 44 so that the armature coils maybe connected to additional circuitry (e.g., electronics that applymotoring currents or draw energy from the flywheel device).

The preferred physical layout of Z-shaped armature coils 44 is shown inFIG. 5 and described in more detail below. Airgap armature coils 44 maybe formed from a unitary piece of solid electrically conductive, lowpermeability material (e.g., copper), but are preferably made up ofturns of wire, each of which may consist of a plurality of electricalconductors that are electrically insulated from each other and areelectrically connected together in parallel. One such wire, known aslitz wire, is constructed of individual film-insulated wires which arebunched or braided together in a uniform pattern of twists and length oflay (thus, a coil formed of litz wire has at least one set of conductorsthat are parallel to each other coupled together in series with at leastone other set of parallel conductors). This configuration reduces skineffect power losses of solid conductors, or the tendency of highfrequency current to be concentrated at the conductor surface. Properlyconstructed litz wires have individual strands each positioned in auniform pattern moving from the center to the outside and back within agiven length of the wire. In addition to the reduction of skin effectlosses, litz wire and other multi-strand bundles of small gauge wireproduce dramatically lower eddy current losses than a single strand oflarger gauge wire.

The effects from the operation of flywheel device 10 are illustrated inFIGS. 2 and 3 as follows. Flywheel device 10 is operated by applying asubstantially DC current (or varying DC current) to field coils 30 and32, which creates a magnetic circuit having a total reluctance thatremains substantially constant while rotor 12 is rotating. The fieldcoils 30 and 32 efficiently induce substantially constant homopolar fluxin the spinning rotor 12 (this flux, which is constant because only DCcurrent is applied to the field coils, does not cause substantialhysteresis and eddy current losses that might otherwise negativelyeffect the performance of the flywheel device). The flux density isproportional to the current in field coils 30 and 32. The induced flux,as indicated by reference numerals 48 and 50, travels radially outwardthrough teeth 14 of rotor 12 (as shown by the arrows at referencenumeral 52 of FIGS. 2 and 3).

The flux passes from rotor teeth 14 through armature gap 46 intolaminated rings 40 and 42, as indicated by arrows 56 (only ring 40 isshown in FIG. 3). The laminated rings 40 and 42 act, in accordance withthe principles of the present invention, to diffuse the flux before itenters shell 18. The flux traveling through flywheel device 10 normallytravels along the path of least reluctance, i.e., along radials directlyout of the teeth 14 and into the shell 18.

Without the flux diffusion that takes place in laminated rings 40 and42, the flux tends to arrive at shell 18 as a high amplitude wave thatfollows the rotation of rotor teeth 14. Considering the total length ofthe magnetic circuit and the high reluctance of extra air gap 54 (ifused), however, the reluctance is substantially the same in the somewhatnon-radial paths shown by the curved arrows in FIG. 3 as in the purelyradial paths. Therefore, the flux diffuses in the laminated rings untilthe flux density is substantially uniform and independent of toothposition by the time the flux passes into unlaminated shell 18. Thisgreatly reduces hysteresis and eddy current losses.

A further improvement on the principles of flux diffusion of the presentinvention may be accomplished, when necessary, by providing additionalair gap 54 between shell 18 and laminated rings 40 and 42. The inclusionof additional air gap 54 permits a radial reduction in the size oflaminated rings 40 and 42, which reduces the core losses due to therotating flux in the laminated rings and reduces manufacturing costs(less laminated material is necessary). The additional air gap 54 actsas a high reluctance barrier that forces flux diffusion to occur in theradially smaller laminated rings. In practice, additional air gap 54 maybe filled with a low permeability material.

Once the flux enters shell 18, the flux changes direction so that it istraveling parallel to the axis of rotation, as shown by arrows 56. Uponreaching the axial limits of shell 18, the flux makes anotherperpendicular change in direction and begins traveling radially towardthe axis of rotor 12 (as shown by arrows 58 and 60). The flux againturns axially (as shown by arrows 62) and crosses axial air gaps 26 and28 (as shown by arrows 64) before reentering rotor 12. The total area ofthe air gaps (whether two air gaps or three are used) remains constantregardless of rotation of rotor 12, and therefore, the total flux in thecircuit does not change with rotation.

In known generators having armature coils embedded in the laminatedring, the discontinuities produced by the slots (or holes) for thearmature windings cause the total flux and reluctance to both varysomewhat with rotation of the rotor, thereby causing higher losses. Bykeeping the total flux and reluctance constant, losses in the solidsteel parts of the circuit are kept small and the circuit remainsefficient (i.e., the generation of heat is kept to a minimum).Additionally, the extra manufacturing required to cut slots in laminatedrings for the armature windings further increases the overall costs ofdevices employing traditional armature windings.

Even though the total flux and reluctance in the circuit remainconstant, the rotating teeth 14 produce a rotating peak flux density inarmature air gap 46 and in the laminated rings 40 and 42. The rotatingpeak flux density in armature air gap 46 induces a current in stationaryarmature coils 44. The output voltage of armature coils 44 is directlyproportional to the tip speed of rotating teeth 14 and to the fluxdensity of armature air gap 46, while power is proportional to thesquare of voltage (given a constant circuit impedance). Because armatureair gap flux density is directly proportional to the current applied tothe field coils 30 and 32, the flywheel device of the present inventionis able to easily maintain a constant output voltage by merely slowlyincreasing the current applied to field coils 30 and 32 as rotor 12slows down. This eliminates the need for expensive power electronicsoften used in conjunction with known energy storage devices. Further, bysimply coupling every third armature coil 44 together, flywheel device10 is able to produce a three-phase alternating current output signal.

A further advantage of the present invention is the fact that flywheeldevice 10 may be used as a three-phase brushless motor with only minorchanges from the previously described configuration. The modificationsrequired for use as a motor are shown more clearly with respect to FIG.5. FIG. 5 shows a preferred configuration of airgap armature coils 44 ofFIGS. 2 and 4. Flywheel device 100 includes a shell 118 of substantiallyhigh permeability material, such as steel, and a single laminated ring140 that is similar to laminated rings 40 and 42 described above. Alsoincluded within shell 118 is a support frame 170 that is formed from anon-magnetic material and a mechanical bearing 172 that holds the shaftof the rotor (not shown in FIG. 5). In the embodiment shown in FIG. 2,support frame 170 may be eliminated if the horizontal portion of shell18 is extended to cover the center of rotation such that shaft 34 may bemounted therein.

Flywheel device 100 includes twenty-four instances of armature coil 44(of FIGS. 2 and 4) mounted about the circumference of laminated ring 140in two offset layers. The preferred armature coils 44 are designed suchthat each of the two vertical legs 174 and 176 are spaced apart to forma window 178 approximately equal to two legs. A lower layer of twelvearmature coils 180 are placed adjacent to each other about thecircumference of laminated ring 140. Then an upper layer of twelvearmature coils 182 are placed on top of the lower layer of coils 180,offset from the lower layer such that the adjacent legs of each lowerlayer pair show through the window 178 of each upper layer armature.Thus, the armatures coils of the present invention are not separated byiron slots (as is traditionally done), and more wire can fit within agiven air gap. The ability to provide an increased volume of wire in theair gap is an additional factor that makes the flywheel devices of thepresent invention more compact, resulting in a higher power density thanpreviously known flywheels.

Although twenty-four armature coils 44 are shown in FIG. 5, personsskilled in the art will appreciate that various other configurations maybe utilized without departing from the scope of the present invention.If the desired device is a three-phase device, however, the total numberof armature coils should be divisible by three in order to maintainproper phase alignment.

Each of the armature coils 44 is also provided with a pair of leads 49from which signals can be input or output. When flywheel device 100 isbeing utilized as an energy output device, the output voltage is simplytaken from leads 49 and efficiently rectified and filtered, asnecessary. In that instance, kinetic energy is efficiently convertedinto electrical energy and flywheel device 100 operates essentially as abattery, but without the use of potentially dangerous chemicals.

To add energy to flywheel device 100, it is driven as a three-phasemotor. In the simplest motor implementation, three additional sensorsare preferably added to the circuit. Hall effect sensors 184, 186 and188 are located such that they monitor the flux density at the left-mostleg of three consecutive armature coils 44. Commutation of each group ofarmature coils (as in the generator configuration, the armature coilsare connected together in three groups) is controlled based on the inputsignals from sensors 184, 186 and 188.

All armatures of a group are driven whenever the sensors sense that anentire left leg is covered by one of teeth 14 (assuming that the teethare rotating from left to right past sensors 184, 186 and 188 across thearmature coils (as shown in FIG. 5)). In this manner, each phase isdriven, in sequence, to continue to drive the rotor about its axis. Oneadvantage to the flywheel device of the present invention is that, whilemany applications require a high power discharge to draw energy from theflywheel, energy may be replenished at a significantly slower rate.Driving power into the flywheel at a slower rate provides a dramaticcost reduction which, by only driving current one way through armaturecoils 44, may be further reduced due to fact that fewer powertransistors are necessary. An improvement in reliability is alsoprovided due to the simplification in required drive logic. The samesensing scheme may also be used with conventional three-phase controllercircuitry to drive armature coils 44 with bi-directional current. It isalso possible to use the back-EMF pulses from armature coils 44 tocontrol motoring commutation without the need for Hall effect sensors.Further, those skilled in the art will appreciate that optical sensorsmay also be used to provide commutation information.

It should also be noted that the energy requirements from energy storagedevices are such that full power is often required in an on-demand mode.To provide such a capability from the flywheel devices described above,the field circuit is preferably kept sufficiently energized at all times(at least in part, due to the delay caused by the inductance of thefield circuit). This requirement, however, produces a small constantpower drain due to field coil heating and core losses in the laminatedring and solid steel parts.

Various features of the present invention, particularly including thesmooth laminated ring without armature coil slots or holes, greatlyreduce these constant standby losses. Nevertheless, if a very largeamount of energy is required, it may be preferable to provide amulti-flywheel system in which only one flywheel is maintained in afully or nearly fully energized state. The energized flywheel must beable to produce enough power to account for the power requirementsduring the time the other flywheels are increased to their fullyenergized states (i.e., full operating flux is being produced). In thiscase, core losses and field coil heating may be minimized while stillproviding for the immediate output of the required power.

FIG. 6 shows an alternate embodiment of the present invention inflywheel device 200. Flywheel device 200 is substantially similar to theflywheel device 10 of FIG. 2. The differences between the two flywheeldevices are the fact that flywheel device 200 has a single laminatedring 202 (versus the two ring configuration shown for device 10) and theconstruction of the shells. While flywheel device 200 is shown with asingle laminated ring 202, persons skilled in the art will appreciatethat the two ring configuration of FIG. 2 may be equally applied toflywheel device 200.

The primary difference between flywheel device 10 and flywheel device200 is the addition of permanent magnets to flywheel 200. Shell 218 isformed from an upper shell 220 and a lower shell 222 that are bothsubstantially similar to upper and lower shells 20 and 22 of FIG. 2.Attached to upper shell 220, however, is upper axially polarized annularpermanent magnet 204, which may be a single magnet, or, due tomanufacturing constraints, may be a set of arc segments fitted togetherto form a ring. An additional annular section 206 of high permeabilitymaterial (preferably steel) is attached to upper permanent magnet 204,while lower axially polarized annular permanent magnet 208 is attachedto section 206. Lower shell 222 connects to lower permanent magnet 208to complete shell 218.

Flywheel device 200 operates in a similar manner to flywheel device 10,however, the inclusion of permanent magnets 204 and 208 eliminates theneed for field current when the flywheel is idling at its top speed inthe standby state. During that time, the permanent magnets 204 and 208act to drive the flux through the magnetic circuit without anyadditional drive current. This feature of the present inventioneliminates the field coil's I² R power losses during standby operationand further increases the reluctance of the field circuit, whichdecreases the inductance of the armatures even further. Even thoughthese improvements are advantageous, the use of permanent magnets maysignificantly impact the overall cost of the device. Thus, underpractical conditions, this permanent magnet embodiment may only beapplicable where the reduction of standby losses and the achievement ofultimate power density is critical. Additionally, while flywheel 200 isshown having additional air gap 54, persons skilled in the art willunderstand that this is merely an option and that the additional gap 54may be eliminated without departing from the spirit of the invention.

A second permanent magnet alternate embodiment is presented in FIG. 7which shows flywheel device 300. Similarly to flywheel device 200,flywheel device 300 is shown having a single laminated ring 202, but, asdescribed above, a dual laminated ring configuration may also beutilized. Flywheel device 300 varies from the flywheels previouslydescribed in that a ring of substantially radially polarized permanentmagnet segments 304 (a single annular permanent magnet ring may also beused) are attached to the inner surface of the laminated ring 202. Theuse of permanent magnet segments 304 provides the same benefits asdescribed above with respect to permanent magnets 204 and 208, however,the magnet 304 may need to occupy a larger volume due to the axialextent of the rotating teeth 14. The larger volume provides anadditional advantage in that a lower energy-product magnet material maybe used (i.e., lower costs). An additional advantage of the radiallypolarized magnets is that they enable additional flux diffusion to occur(assuming the magnets are made of a low permeability, low conductivitymaterial), while also further reducing the inductance of the armaturecoils to allow still higher power density. Further, as described above,the inclusion of additional air gap 54 is optional.

In addition to the dual field coil configurations described above, theprinciples of the present invention may be practiced through the use ofseveral single field coil configurations as shown in FIGS. 8-11. FIG. 8shows flywheel device 400 that includes a rotor 412 having teeth 414 asdefined by dashed line 416 and described above, as well as a lowerportion 436. Rotor 412 rotates within a stationary shell 418 (formedfrom a single piece of high permeability material such as steel). Asingle field coil 430 induces unidirectional axial homopolar flux inlower portion 436 upon the application a substantially DC current. Theflux travels from lower portion 436 to the teeth 414 of rotor 412 asshown by arrows 438.

The flux travels in an outward radial direction upon reaching theapproximate top of rotor 412 and exits rotor 412 through teeth 414 aspreviously described for flywheel 10 (with respect to rotor 12 and teeth14). Upon exiting teeth 414, the flux crosses armature gap 446 andpasses through armature coils 444 into single laminated ring 440.Laminated ring 440 acts to diffuse the flux before the flux enters shell418 in the same manner as previously described. The flux then travelsaxially through shell 418 before turning perpendicularly toward the axisof rotor 412. Finally, the flux traverses a small air gap 428 beforereentering rotor 412.

As previously described, the total flux and reluctance in the circuit donot change with the rotation of rotor 412, however, the rotating teeth414 do produce a local changing flux density in the armature air gap 446and in the laminated ring 440 as it passes through. As previouslydescribed with respect to FIG. 4, the leads of airgap coils 444 may beconnected to external circuitry that causes the armature to interactwith this changing flux density to convert the flywheel's kinetic energyto electrical energy or to convert externally supplied electrical energyinto kinetic energy. The components of flywheel 400 have essentially thesame properties as those previously described for flywheels 10, 100, 200and 300 with respect to proportions between induced voltage, changingflux, flux density and current in the field coil.

FIG. 9 shows an alternate embodiment of a single field coil flywheeldevice 500 that includes substantially all of the components of flywheeldevice 400. For simplicity, each of the components that is essentiallyidentical between flywheel devices 400 and 500 has the same last twodigits of the reference numeral and therefore, the discussion aboveapplies equally to them (and they are not discussed further). Theprincipal difference between flywheels 400 and 500 is the inclusion of apermanent magnet 504, in a manner similar to that previously describedwith respect to FIG. 6. Shell 518 is formed of an upper shell 520 thatis attached to magnet 504, and magnet 504 is attached to lower portion522. Even though flywheels 400 and 500 are shown without an additionalair gap between the laminated stack and the shell, persons skilled inthe art will appreciate that the additional air gap may also be usedhere.

FIG. 10 shows another alternate embodiment of a single field coilflywheel device 600 that includes substantially all of the components offlywheel devices 400 and 500. For simplicity, each of the componentsthat is essentially identical between flywheel devices 400, 500 and 600has the same last two digits of the reference numeral and therefore, thediscussion above applies equally to them (and they are not discussedfurther). The principal difference between flywheels 500 and 600 is thelocation of the permanent magnet 604. Flywheel device 600 employs thepermanent magnet 604 in a manner similar to that previously describedwith respect to FIG. 7. Shell 618 is formed of a single piece of highpermeability material and the permanent magnet 604 as attached tolaminated ring 640. Further, flywheel 600 may also have an additionalair gap between the laminated stack and the shell even though none isshown.

FIG. 11 shows another alternate embodiment of the present invention inflywheel device 800. Flywheel device 800 provides a reduction in averagestand-by losses. This advantage comes in exchange for an increase inmechanical complexity, a decrease in stored energy or a reduction insafety factor. As such, flywheel 800 may have to operate at lowerrotational speeds than flywheels 10, 100, 200, 300, 400, 500 and 600 toachieve the same level of safety. Therefore, flywheel 800 may require aheavier flywheel to store the same energy as the other flywheels.

Flywheel 800 includes a rotor 812 having teeth 814 as defined at dashedline 816. Rotor 812 also includes a lower portion 836 that extendsradially to the beginning of teeth 814. The lower portion 836 of rotor812 is connected to a low-permeability disk 824 so that it rotates asrotor 812 rotates. The outer portion 820 of shell 818 is physicallyconnected to non-ferrous disk 824 so that it also rotates insynchronization with rotor 812. An additional lower portion 822 of shell818 is stationary and is fixed to annular field coil 830. Thus, in FIG.11, only lower portion 822 and field coil 830 are stationary duringnormal operations. Lower portion 822 is positioned such that there aretwo gaps 826 and 828 on either side of portion 822. Outer portion 820 ispositioned such that an armature gap 846 is formed between outer portion820 and rotor 812. Armature coils (not shown) located in air gap 846 arepreferably used to connect flywheel device 800 to other electricalequipment.

One of the advantages of flywheel 800 is the elimination of thelaminated ring as a diffusion element. Instead, outer portion 820 isavailable for flux diffusion because the entire height of portion 820tends to substantially homogenize the flux so that it is essentiallyequal at all circumferential positions by the time it enters lowerportion 822.

While each of the above-described embodiments are illustrative of fluxthat passes radially through the armature coils, persons skilled in theart will appreciate that the principles of the present invention applyequally to embodiments in which the flux passes axially through thearmature. One example of an axial flux embodiment is shown in FIGS. 12and 13 by dual field coil flywheel device 900. Additionally, althoughflywheel device 900 is shown as a dual field coil device, personsskilled in the art will also appreciate that flywheel device 900 may beimplemented with a single field coil (and associated armature coil)located either above or below rotor 912 without departing from thespirit of the present invention.

Flywheel device 900 operates on essentially the same principles as thepreviously described flywheel devices, except that the relative geometryis configured such that flux passes axially through the armature coilsrather than radially. As such, and for simplicity, each of thecomponents that is essentially identical between the previouslydescribed flywheel devices and flywheel device 900 has the same last twodigits of the reference numeral and therefore, the discussion aboveapplies equally to them (and they are not discussed further). Whileflywheel device 900 is preferably equipped with Z-shaped nestedthree-phase armature coils, similar to the previously discussed armaturecoils, armature coils 984 and 986 are shown as flat coils suitable forsingle phase output (compare the coils shown in FIGS. 4 and 5 with thecoil shown in FIGS. 12 and 13).

Persons skilled in the art will appreciate that the single phase flatcoils shown in FIGS. 12 and 13 may be used with any of the previouslydescribed embodiments of the invention without departing from the scopeof the invention. Flat coils, which are simpler and less expensive toproduce than Z-shaped coils, tend to reduce the power density of thegenerator, which may not always be a significant design consideration.Additionally, because the flux is axially traveling through laminatedrings 940 and 942, rings 940 and 942 may also be produced at reducedcosts (e.g., rings 940 and 942 may be produced by simply winding a coilof flat steel sheet onto a mandrel, which eliminates that part of thesteel that is wasted from being cut out of previously describedlaminated rings, whether they be made from uninterrupted rings or arcsegments).

Flywheel device 900 operates similarly to the previously describedflywheel devices in that field coils 930 and 932 are driven by asubstantially DC current. In the configuration of FIG. 12, however, theinduced flux exits the teeth 914 axially, rather than radially from thetip of the teeth 914. The flux then travels through the airgap armaturecoils 984 and 986, and through the laminated rings 940 and 942, beforeentering the split shell 918. Although FIG. 12 shows a pair ofadditional air gaps 954 and 955, persons skilled in the art willappreciate that the gaps may be omitted, or they may be filled with alow permeability material, as previously described, without departingfrom the spirit of the present invention. Additionally, the principlesof the present invention may be practiced by combining the embodimentshown in FIG. 12 with permanent magnets in any of the manners describedabove to change operational parameters of the flywheel device.

The configuration of flywheel device 900 provides the designer withadditional factors to consider in making design selections. For example,flywheel device 900 has a smaller overall diameter than the previouslydescribed flywheel devices at the cost of slower average tip speed inair gaps 954 and 955 (for a given diameter). The smaller diameter alsoresults in the non-rotating components of the magnetic circuits beinglighter and being produced at lower costs.

FIG. 14 shows a representative installation of flywheel device 960 thatmay correspond to any of the flywheel devices described above (i.e.,rotor including a single set of teeth and a laminated ring). As shown,however, FIG. 14 particularly applies to embodiments of the presentinvention that include a lower portion X36 as part of the rotor. Personsskilled in the art will appreciate that the principles of the inventionshown and described with respect to FIG. 14 are equally applicable tothose embodiments in which the rotor is simply a flat disk (with minormodifications to the installation shown in FIG. 14). As such, only theinstallation components are discussed with respect to FIG. 14. The shaft34 of the rotor rests in two mechanical bearings 962 and 964. Mechanicalbearings 962 and 964 are preferably simple ball bearings or rollerbearings because they are cheap, simple and reliable.

In accordance with the principles of the present invention, thecharacteristics of the mechanical bearings 962 and 964 are greatlyimproved by reducing the loading on the bearings. Reduced load enablesthe bearings 962 and 964 to perform with minimum levels of drag and haveextended service lives. Longest life, however, may be obtained from themechanical bearings by insuring that some small load is maintained onthe bearings, rather than substantially removing the entire load.

The load on mechanical bearings 962 and 964 is reduced through the useof a magnetic bearing in the form of annular electromagnet 966. One ormore strain gauges (not shown) that provide inputs to a closed-loopcontroller (not shown) may be located on a support structure within theshell. The controller commands the electromagnet 966 to support amajority of the weight of the rotor while maintaining an optimumconstant load on mechanical bearings 962 and 964. If the bearing andelectromagnet supports are of adequate stiffness, the magnetic bearingmay be controlled by a simple constant current, and no strain gauges arerequired. The fact that the rotor is a solid magnetic component enablesthe electromagnet 966 to operate directly on the rotor (rather than on aseparate steel rotor as in previously known composite flywheel systems).Persons skilled in the art will appreciate that electromagnet 966 ismost effective when flywheel device 960 is installed such that shaft 34is substantially parallel to earth's gravity vector 968.

FIG. 15 shows a representative example of how the principles of thepresent invention may be applied to provide an uninterruptable powersupply system 970. System 970, which receives primary power at IN(typically from a power company), and provides supply power at OUTincludes a flywheel storage unit 972 that may be any flywheel energyconversion device that includes a field controllable generator toprovide short-term back-up power. System 970 includes at least one ofthe following: input line monitor 974, output line monitor 976 and DCbuss monitor 978--any or all of which may serve to directly orindirectly monitor disruptions of the primary power, field coilcontroller 980, rectifiers 982 and inverter 984 (inverter 984 includestransistor timing and driving circuitry (not shown)). If longer termemergency power is required--i.e., longer than can be supplied by thekinetic energy stored in flywheel storage unit 972--transfer switch 986may be included to transfer the supply lines to standby power source988, such as a standby diesel generator.

Under normal operating conditions AC power is input from IN torectifiers 982 which convert the power to DC. A small portion of the DCpower is converted back into AC power by a small inverter (not shown)and provided to flywheel storage unit 972 to accelerate the flywheel toits nominal standby rpm, and to keep the flywheel spinning at itsnominal standby rpm (i.e., to overcome the electrical and mechanicalstandby losses described above). To minimize losses, the field coil inflywheel storage unit 972 is preferably kept at a reduced value until aprimary power failure is detected. The majority of DC power is fed toinverter 984 which converts the power back to AC and provides it toexternal circuity (not shown) through OUT.

Once a primary power disruption is detected by any of monitors 974, 976or 978, field coil controller 980 sends a drive signal to the field coilin flywheel storage unit 972 that causes the field coil current torapidly ramp up such that power required at OUT is provided by flywheelstorage unit 972 (the present invention rapidly ramps up the field coilcurrent such that field coil responds to the increased drive signalwithin substantially less than one second of the disruption in primarypower). If necessary, a capacitor or other secondary storage device (notshown) may be used to provide power to maintain the buss voltage and topower the field coil during ramp up time, which should only take severalmilliseconds.

An additional feature of the present invention is that monitoringcontinues as power is supplied by flywheel storage unit 972 so that theoutput voltage is maintained at a relatively constant level until aboutninety percent of the kinetic energy has been depleted from theflywheel. This may be accomplished by monitoring any of: the outputvoltage of flywheel storage unit 972, the output voltage at OUT, or therotational speed of the flywheel. As kinetic energy is removed from theflywheel (i.e., it begins to slow down) and the output voltage offlywheel storage unit 972 begins to drop, field controller 980 slowlyramps up the field current being provided to the field coil whichreestablishes and maintains the output voltage of storage unit 972 at aconstant level. This technique becomes less effective as the amount ofstored energy is reduced until, when approximately ten percent of thestored energy is remaining, the output voltage begins to drop becausethe field coil current has reached its maximum value and can no longercompensate for the rotor's decreasing rpm.

It will be understood that, in circumstances where the advantage ofmaintaining constant output voltage as rpm decreases is not required,the other advantages of the present invention may be achieved by usingpermanent magnets to drive the magnetic circuit instead of field coils(at a significantly increased cost, however, due to the use of expensivepermanent magnet material). In such circumstances, the field coils wouldbe entirely omitted, thus saving space and achieving a slight reductionin the amount of increased costs that will be incurred due to thepermanent magnet material.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the advantages of a rotating steel shellportion may be applied to the non-extended rotor (i.e., rotor withoutthe lower portion as shown in FIGS. 2-7) by utilizing the principlesdisclosed with respect to FIG. 11 even though that individualcombination of components was not specifically described.

We claim:
 1. An airgap electrical machine comprising:a stationarymember; a rotatable member rotatably mounted within said stationarymember such that an air gap is established between said stationary androtatable members; and a plurality of airgap armature coils thatsubstantially fill said air gap, each airgap armature coil including:atleast one electrical conductor that is formed to create a substantiallysolid shape having two legs and two cross members, said two legs beingsubstantially parallel to each other and spaced apart from each other,said cross members being bent such that said legs and said cross membersare in substantially different planes from each other, said crossmembers and said legs being separated to form a window having acircumferential width about equal to the circumferential width of twolegs, said window being such that when two adjacent armature coils areplaced over a single armature coil, the window of the single armaturecoil is substantially filled by the two adjacent legs of said twoadjacent armature coils; said rotatable member having a plurality ofintegral protrusions extending therefrom wherein the circumferentialdistance between said protrusions is approximately equal to thecircumferential distance between the two legs of said armature coil,said rotatable member having an axial thickness that remainssubstantially constant or decreases when measured at increasing radialpositions; and first and second leads being connected to said at leastone conductor such that said conductor can be connected to externalcircuitry.
 2. An airgap electrical machine comprising:a stationarymember; a rotatable member rotatable mounted within said stationarymember; said stationary member including at least one ring having asubstantially smooth surface, said ring being mounted concentric withrespect to said rotatable member such that an armature air cap is formedbetween said rotatable member and said smooth surface, said ring beingconstructed of substantially high Permeability material; a plurality ofairgap armature coils that substantially fill said air gap, each airgaparmature coil including:at least one electrical conductor that is formedto create a substantially solid shape having two legs and two crossmembers, said two legs being substantially parallel to each other andspaced apart from each other, said cross members being bent such thatsaid legs and said cross members are in substantially different planesfrom each other, said cross members and said legs being separated toform a window having a circumferential width about equal to thecircumferential width of two legs, said window being such that when twoadjacent armature coils are placed over a single armature coil, thewindow of the single armature coil is substantially filled by the twoadjacent legs of said two adjacent armature coils; and first and secondleads being connected to said at least one conductor such that saidconductor can be connected to external circuitry; and a member thatgenerates homopolar flux said rotatable member and said flux generatingmember forming a magnetic circuit having a total reluctance that remainssubstantially constant while said rotatable member is rotated about anaxis.
 3. The airgap electrical machine of claim 2, wherein said fluxgenerating member comprises at least one permanent magnet that inducessaid flux to flow.
 4. The airgap electrical machine of claim 2, whereinsaid flux generating member comprises at least one field coil thatinduces said flux to flow.
 5. The airgap electrical machine of claim 4,wherein said at least one field coil comprises a pair of upper and lowerfield coils, said upper field coil being mounted above said rotatablemember and said lower field coil being mounted below said rotatablemember.